The present invention relates to pottery materials which comprise fine foamed particles called shirasu balloons made through firing and expansion of white or graysh white volcanic ejects and the method of manufacturing the pottery products, together with constitution of the products.
The term shirasu which denotes a kind of volcanic ejecta which makes fine foamed particles in a process described later is well known in Japan and is used as an established technical term together with shirasu balloon.
It is also known that the chemical composition and the component proportions of shirasu are as shown below, while depending on the district in which it is unearthed. SiO.sub.2 (65-73), Al.sub.2 O.sub.3 (12-16), Fe.sub.2 O.sub.3 (1-3), CaO (2-4), Na.sub.2 O (3-4), K.sub.2 O (2-4), TiO.sub.2 (0.2-0.7), MgO (0.4-1).
As can be seen from the chemical composition and component proportions, shirasu has such advantages as light-weight, excellent fire resistance and heat insulation, low water absorption and low cost, and is therefore looked at as a useful new material for various industrial applications.
For these advantages, shirasu has recently been put into industrial applications as a promising new material, and a product made by firing ground shirasu, then heating it so that the particles expand and foam, is marketed as shirasu balloon.
Shirasu balloon has been employed in many industrial applications such as construction materials including concrete aggregate for weight reduction, top coating mortar, top coating plaster and sprayed coating material, and agricultural purposes including seedbed material and soil modifier.
Laid open Japanese patent gazette No. S55-116651, No. S62-98592 and No. S63-203555 show that shirasu balloon has been applied to make tiles, microwave oven trays and tableware to reduce the weight of these products.
The present inventors tried to make light-weight pottery products taking advantage of the properties of shirasu balloon.
However, shirasu includes impurities such as iron and titanium as it is a natural material, and therefore has drawbacks such as staining on the product surface and difficulty in obtaining clear white color because of oxidation of the iron contents during firing after being glazed, in spite of its advantage of being capable of reducing the weight of pottery products, due to its physical properties.
Thus it is indispensable to remove impurities to make white pottery products, although said process is not required in the manufacture of non-white pottery products.
Furthermore, the difference in the bulk density between shirasu balloon, the body and other materials used to manufacture pottery products makes it difficult to uniformly mix shirasu balloon with the body and other materials to make pottery.
The inventors also found through an experiment that the use of shirasu balloon decreased the strength of pottery products.
The primary objective of the invention is to remove impurities such as iron and titanium from shirasu balloon to make it applicable to the manufacture of pottery products.
The second objective of the invention is to provide a means of making shirasu balloon diffuse uniformly among the body and other materials.
The third objective of the invention is to provide a means of increasing the strength of pottery products which include shirasu balloon without compromising the advantages of using shirasu balloon.
The merits set forth above and other merits of the invention will become evident through the detailed description of the invention that follow, with reference to the drawings.
Identical numbers used in different figures denote similar objects.